Silica-based optical fibers and multi-pass sintering

ABSTRACT

A process produces a glass overcladding tube from a silica gel body. The process includes passing the gel body through a hot zone under conditions that cause partial sintering of the gel body and repassing the gel body through the hot zone under conditions that further sinter the gel body into a glass overcladding tube.

BACKGROUND OF THE INVENTION

[0001] This application claims the benefit of the U.S. ProvisionalApplication No. 60/222,444; titled “Silica-Based Optical Fibers AndMulti-Pass Sintering” by Richard M. Lum, David A. Mixon, Eric M.Monberg, and Dennis J. Trevor; and filed Aug. 1, 2000.

FIELD OF THE INVENTION

[0002] This invention relates to optical fibers and fabrication ofpreforms for use in drawing optical fibers.

DISCUSSION OF THE RELATED ART

[0003] Contemporary optical fibers are drawn from a cylindricalsilica-glass object generally referred to as a preform. The preform hasan axially symmetric structure that reflects the final structure of theoptical fiber. The preform's structure usually includes a central core,a middle cladding layer, and an outer overcladding or jacketing layer.To achieve the desired optical properties of the fiber, the core has ahigher index of refraction than the cladding layer. Differences inindexes of refraction of the various fiber layers come from dopants,e.g., germanium and/or fluorine, which are incorporated duringproduction of the preform.

[0004] As preform size continues to increase, in order to reduce fibercosts, the amount of overcladding relative to cladding also increases.The overcladding may comprise more than 85 percent of the fiber'svolume. The overcladding and its interface with the core-claddinglargely determine mechanical properties such as draw breaking frequencyand fiber limpness, i.e., curl. Though the overcladding determinesmechanical properties, the core and cladding carry about 99 percent ofthe optical energy and primarily determine the fiber's opticalproperties. The diminished impact of the overcladding on opticalproperties suggests fabricating the overcladding with processes thatproduce high mechanical quality but lower optical quality. Since suchprocesses are often less costly, using them to fabricate the voluminousovercladding can substantially reduce overall production costs forpreforms and for final optical fibers.

[0005] A sol-gel process is described in U.S. Pat. No. 5,240,488,(“'488”), which is incorporated by reference herein in its entirety. Bythe sol-gel process, overcladding tubes can be fabricated more cheaplythan by processes using deposited soot as starting material. Fabricationof an overcladding tube using the sol-gel process involves casting aporous and opaque gel body from a colloidal sol of silica particles. Thegel body is then dried, purified and sintered to produce the finalsilica-glass overcladding tube. A pre-made rod structure for the coreand cladding is inserted into the overcladding tube, which is collapsedto produce the final preform.

[0006] In the sol-gel process, the treatment of the dried gel body hasat least two stages. In a first stage, a purification treatment removesimpurities, e.g., organic matter, water, and transition metals. Theseimpurities are either present in the fumed silica starting material orin additives used to produce the gel body or are contaminants introducedduring processing. In a second stage, a heat treatment sinters the gelbody to close pores between silica particles and produce the final glassovercladding tube from the porous gel body. Herein, sintering is definedas a heat treatment that causes a measurable shrinkage in a gel body'slinear dimensions, e.g:, a diameter or length, of at least one percent.

BRIEF SUMMARY OF THE INVENTION

[0007] A first embodiment features a process that produces a glassovercladding tube from a silica gel body. The process includes passingthe gel body through a hot zone under conditions that cause partialsintering of the gel body and repassing the gel body through the hotzone under conditions that further sinter the gel body into a glassovercladding tube.

[0008] A second embodiment features another process for producing aglass overcladding tube from a silica gel body. The process includessubjecting one end of a cylindrical silica gel body to a hot zone untilthe end is at least partially sintered. The process also includesvertically passing the gel body through the hot zone to sinter the gelbody. The act of passing causes the partially sintered end to enter thehot zone last.

[0009] Another embodiment features a manufacture for a preform. Thepreform has a core, a cladding layer, and an overcladding layer. Thecore, cladding layer, and overcladding layer each include silica-glass.The preform has an OD variation of 0.1 percent or less at onelongitudinal position along the length of the preform.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0010]FIG. 1 is a cross-sectional view of one embodiment of a sinteringapparatus;

[0011]FIG. 2 is a flow chart illustrating one embodiment of a multi-passprocess for sintering silica gel bodies;

[0012]FIG. 3 is a graph showing shrinkages of an exemplary gel bodyduring multiple-pass sintering; and;

[0013]FIG. 4 is a flow chart illustrating another embodiment of aprocess for sintering silica gel bodies; and

[0014]FIG. 5 is a flow chart illustrating one embodiment of a processfor fabricating preforms.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Various embodiments sinter large gel bodies into silica-glassobjects, such as overcladding tubes, incrementally by processes thatreduce stress levels on the gel body below stress levels encounteredduring conventional sintering processes. Recently, conventionalprocesses were used to sinter large gel bodies to overcladding tubes forpreforms from which about 600 kilometers of single-mode optical fibercan be drawn. For the large overcladding tubes, initial tubular silicagel bodies had lengths, outer diameters (ODs), and inside diameters(IDs) of about 1600, 120, and 43 millimeters (mm), respectively.

[0016] The sintering of these large cross-sectional area (CSA) gelbodies was accompanied by several problems. First, the last-to-sinterends of the large gel bodies tended to fracture with conventionalsintering procedures. Second, overcladding tubes made from the large gelbodies had less uniform inside diameters (IDs) and CSAs, which degradesthe ability to physically match such overcladding tubes to core-claddingrods thereby increasing dispersion variations among the final fibersproduced from such tubes. Dispersion is a critical performance fiberparameter for many applications. Third, the overcladding tubes made fromthe large gel bodies had high variations in [OH] levels, e.g., fromabout 5 to 30 parts per million (ppm). High [OH] levels are undesirablefor overcladding tubes used to make optical fibers that will transmitlight with any wavelength between about 1.55 and 1.31 microns, i.e., arange containing a strong [OH] optical absorption peak.

[0017]FIG. 1 shows a sintering apparatus 10 for producing a silica-glassovercladding tube from a tubular silica gel body 12. Silica gel bodiesare porous and opaque to visible light. The silica gel body 12 is madeby one of the sol-gel processes described in the '488 patent. The gelbody 12 may contain residual impurities such as organic materialsadsorbed onto the gel body 12 subsequent to purification andpurification byproducts that may remain in the body's pores due to theirlow volatility.

[0018] In some embodiments, the silica gel body 12 may have anothershape and may be prepared by other processes. For example, the gel body12 may have a shape adapted for producing lenses, prisms, or silicaflanges or fixtures of diverse shapes. Such gel bodies can be producedfrom aerogels, alkoxide-based gels, or xerogels known to those of skillin the art.

[0019] During sintering, the gel body 12 is enclosed in acontrolled-atmosphere muffle 14, e.g., a fused quartz firing shroud withan end plate or a furnace liner. The muffle 14 has a port 16 forintroducing gases into and a second port for exhausting gases from theregion adjacent the gel body 12. One opening 18 into the muffle 14allows attaching a mechanical device 20 that supports the weight of thegel body 12 during sintering. For example, the device 20 may be the topsupport described in co-pending U.S. patent application Ser. No.09/459,775, filed Dec. 13, 1999, which is incorporated herein byreference.

[0020] The sintering apparatus 10 can vertically raise or lower the gelbody 12 through a hot zone 22 of a furnace 24 at an adjustable speed sothat the gel body 12 passes through the hot zone 22. The length of themuffle 14 accommodates raising and lowering the gel body 12 completelythrough the hot zone 22.

[0021] The temperature of the hot zone 22 can be gradually andcontrollably varied between about 0-1,600° C. by a control apparatus 26.During sintering, portions of the gel body 12 are heated to temperaturesbetween about 1350 and 1600° C. This initiates viscous sintering causingthe gel body 12 to shrink and finally transform into a transparentsilica overcladding tube. For a given furnace configuration, the gelbody's CSA and the traversal rate through the hot zone 22 will determineaxial and radial temperature gradients within the gel body 12. Duringsintering, temperature gradients may produce large stresses in the gelbody 12. The stresses induced in the gel body can increase the chancesof cracking during the sintering or subsequent processing. Performingthe sintering incrementally can lower such stresses.

[0022]FIG. 2 is a flow chart showing a process 30 for multi-passsintering of silica gel bodies, e.g., using sintering apparatus 10 ofFIG. 1. Initially, the process 30 causes the gel body to pass along thehot zone partially sintering the gel body (step 32). Passing the bodyalong hot zone may entail raising the gel body vertically up through thehot zone in a direction opposite to gravity or lowering the gel bodyvertically down through the hot zone in the direction of gravity.Alternatively, passing the gel body along the hot zone may entail movingthe hot zone instead of the gel body so that the hot zone passes overthe gel body. During the sintering, the furnace's hot zone is kept at ahigh enough temperature to cause closure of pores between silicaparticles and shrinkage of the gel body, e.g., 1300-1550° C. Partialsintering occurs if shrinkage reduces linear dimensions of the gel body,e.g., the diameter and length, by 1% or more and may cause shrinkage ofthese dimensions by 5% or more. The extent of shrinkage depends on thetime in the hot zone, temperature of the hot zone, pore size in the gel,and the viscosity of the silica. After the first partial sintering pass,the process 30 causes the gel body to pass through the hot zoneproducing further sintering of the gel body (step 34). The furthersintering incrementally shrinks linear dimensions of the gel body by onepercent or more. By incrementally sintering the gel body through two ormore steps, the process 30 decreases mechanical stresses with respect toconventional processes that entirely sinter the gel body in one pass.Performing sintering incrementally in several passes lowers risks thatthe gel body will crack during sintering.

[0023] In the process 30, each incremental sintering pass shrinks lineardimensions of the gel body by a fraction of the total shrinkage neededto fully sinter the gel body. About a 24 percent total shrinkage isgenerally needed to produce the transparent overcladding tube from a gelbody prepared via a sol-gel process such as described in '488. Thesintering-induced volume shrinkage of process 30 is more gradual thansingle pass processes and produces lower cracking stresses in the gelbody. One embodiment of the process 30 performs three sintering passesthrough the hot zone to shrink the gel body's diameter by a total ofabout 8, 16, and 24 percent after the first, second, and third sinteringpasses, respectively (see FIG. 3). In this embodiment, the successivesintering passes are performed at successively higher furnacetemperatures between about 1300 and 1580° C. The temperature is nothowever, raised after each sintering pass in all embodiments.

[0024] Each pass may vertically pull the gel body 12 up through the hotzone 22 opposite to the direction of gravity so that the top of the gelbody 12 is sintered first. Sintering the top first reduces theprobability of a catastrophic crack. Cracks usually form at the lastsintered end of the gel body 12, because the last-to-sinter end issubject to the higher sintering stresses. If the gel body 12 is pulledup through the hot zone 22, a crack is more probable to form near thebottom of the gel body 12, because the bottom is the last portion tosinter. Then, only the bottom of the gel body is likely to crack andbreak off if the sintering-induced expansion stresses become too large.

[0025] Each pass may alternatively lower the gel body 12 verticallydown, in the direction of gravity, through the hot zone 22. In thiscase, a crack is more probable to form near the top of the gel body 12,because the top becomes the last portion to sinter. Then, cracks aremore likely to form near the top of the gel body 12. Such a crack couldbe catastrophic and cause the whole gel body to break off top supportingmechanical device 20 completely destroying the gel body and possiblydamaging the furnace 24.

[0026] One embodiment sinters a silica gel body having an initial lengthof about 1600 mm or more, an OD of about 120 mm or more, an ID of about43 mm or less, and a weight of about 14 kilograms or more in threesintering steps. The three steps produce a total reduction of the OD andlength by about 24 percent. During each sintering step, the atmospheresurrounding the gel body 12 is an oxygen and helium mixture. For thesintering steps, which cause pore closure, the molar ratio of oxygen tohelium is less than or equal to about 0.025. For these steps, a higheroxygen percentage can cause bubble formation due to the low diffusivityof trapped oxygen. Bubbles in the glass can cause air lines in the fiberdrawn from a preform using the overcladding tube or frothing of theovercladding tube itself. The low partial pressure of oxygen aids toremove organic impurities and to oxidize the Si—Cl moiety created duringa previous dehydroxylation step. The oxygen combines with the Si—Clmoiety to form siloxane bonds, i.e., Si—O—Si, and release chlorine gas.

[0027] In the exemplary embodiment, the temperature is about 1380° C.during the first sintering pass, and the gel body is vertically pulledup through the hot zone 22, against gravity, at a rate of about 30 mmper minute. The pass produces substantial shrinkage of the gel body 12.After the entirely passing through the hot zone 22, the gel body 12 israpidly lowered back through the furnace 22, e.g., at a rate of 500 mmor more per minute, to reposition the gel body 12 for the next pass. Thelowering rate is fast enough to not produce substantial sintering orshrinkage.

[0028] For the next sintering pass, the temperature is ramped up to ahigher temperature of about 1440° C., and the gel body is verticallypulled up through the hot zone 22, against the pull of gravity, at therate of about 30 mm per minute. The higher temperature decreases theprocessing time needed to produce further sintering. During the secondpass, chlorine gas, water and volatile compounds continue to dischargefrom the gel body 12 as impurities are further removed. After entirelypassing through the hot zone 22, the gel body 12 is rapidly lowered backthrough the furnace 22 to reposition the gel body 12 for the next pass.

[0029] For the third sintering pass, the temperature of the furnace 24is raised further to about 1500° C., and the gel body is pulled upthrough the hot zone 22, against gravity, at a slower rate of about 10mm per minute. This last pass produces further shrinkage and completessintering to produce the transparent overcladding tube. After the lastpass, the gas mixture is changed to pure helium, and the silica-glasstube is cooled down to 25° C. over a period of about an hour.

[0030]FIG. 3 indicates data points 35-38 for shrinkages of the OD of onetubular gel body during individual passes of the gel body through a hotzone of a sintering furnace. The first three passes lifted the gel bodythrough the sintering furnace at rates of about 30 mm per minute. Thefinal pass lifted the gel body through the sintering furnace at a rateof about 10 mm per minute. The data point 37 below 1350° C. correspondsto a purification pass of the gel body through the hot zone in whichsintering does not occur, i.e., less than one percent shrinkage of thebody's diameter and length. The last data point 38 corresponds to thefinal transparent overcladding tube for which the diameter of theinitial gel body has undergone a total shrinkage of about 24 percent.

[0031]FIG. 4 is a flow chart showing an alternate process 40 formulti-pass sintering of silica gel bodies, e.g., using apparatus 10 ofFIG. 1. Initially, the process 40 performs an end dip by subjecting anend portion of the gel body to the furnace's hot zone to partially orfully sinter that end portion (step 42). For example, the process 40 maylower 20-100 mm of the gel body into a 1500-1540° C. hot zone at a rateof 5-50 mm per minute during the end dip. The resulting heat treatmentcauses shrinkage of the end of the gel body that is indicative ofsintering, i.e., shrinkage of the diameter by 1% or more. Afterpartially or fully sintering the end portion, the tube is rapidlylowered vertically through the hot zone until the top of the gel body isat the center of the hot zone. Then, process 40 pulls the entire gelbody, opposite to the direction of gravity, vertically up so that thegel body passes through the furnace's hot zone in a manner that sintersthe entire gel body and sends the partially sintered end portion throughthe hot zone last (step 44). This complete sintering pass of the gelbody through the hot zone shrinks the diameter of the gel body by about23-27 percent. During this sintering pass, lower stresses are exerted onthe end portion of the gel body due to the previous sintering of thatportion. The lower stresses at the last-to-sinter end reduce risks ofcrack formation during the complete sintering pass, because cracks tendto propagate out from the last-to-sinter end of the gel body.

[0032] The multi-pass process 30 and the end-dip process 40 use similaramounts of time to sinter a silica gel body.

[0033]FIG. 5 is a flow chart for a process 50 that fabricates preformsfor drawing single-mode or multi-mode optical fibers. The process 50includes preparation of a porous silica gel body (step 52). The gel bodymay be formed by the sol-gel process, which molds a silica gel body froma sol of silica particles and then dries the gel body to remove 95-98%of the water initially present therein as described in the '488 patent.The gel body may also be formed from an aerogel, an alkoxide-based gel,or an xerogel, which has been dried, for example, through a microwaveprocess.

[0034] After drying, the gel body may still have contaminants, e.g.,quaternary ammonium salts, organic polymers, metal oxides and transitionmetals. To remove these contaminants, the process 50 performs apurification and dehydroxylation treatment of the gel body in amoderate-temperature furnace, i.e., below 1000° C. (step 54).

[0035] The purification and dehydroxylation treatment includes severalstages. The first exemplary stage heats the dried gel bodies to about350° C. in a bath of nitrogen gas to decompose quaternary ammonium saltsreleasing gaseous byproducts. The next stage changes the atmosphere toair so that oxygen therein reacts with and decomposes the organicimpurities releasing gaseous byproducts. The next stage changes theatmosphere to thionyl chloride, which reacts with the refractory metaloxides releasing gaseous byproducts. The last stage changes theatmosphere to chlorine and raises the temperature to about 950° C. Thechlorine dehydroxylates the gel body by reacting with silica hydroxidesto produce silicon-bound chlorine and gaseous byproducts. The gaseousbyproducts are removed.

[0036] After the purification and dehydroxylation treatment, the gelbodies may still have residual impurities including chemically boundchlorine (bound during dehydroxylation), metal chlorides, and organicmaterials adsorbed during any storage period. To remove these residualimpurities, the process 50 passes the gel body through a sinteringfurnace's hot zone, e.g., the hot zone 22 of FIG. 1 (step 56). Thispurification pass does not significantly close pores between silicaparticles, because the hot zone is kept below about 1300° C. or at leastbelow about 1350° C. Shrinkage of the diameter or length of the gel bodyby less than about 1 percent is indicative of insignificant pore closureand characteristic of the purification pass through the hot zone.

[0037] The purification pass through the hot zone is performed in anatmosphere of oxygen and helium in which the molar ratio of oxygen tohelium may be between 0.5 and 0.025. High molar fractions of oxygen areallowed, because the purification pass does not result in the closing ofpores and the subsequent trapping of oxygen. The oxygen reacts with thebound chlorine to produce silica glass and chlorine gas. The chlorine isflushed out of the muffle surrounding the gel body. The oxygen alsooxidizes adsorbed organic impurities to produce other gaseous byproductsthat are flushed out.

[0038] For the above-described 1600 mm long silica gel body, oneembodiment pulls the gel tube vertically up through a hot zone heated toabout 1320° C. at a pull rate of about 30 mm per minute to perform thepurification pass. After passing through the hot zone 22, the gel body12 is rapidly lowered back through the furnace 22, e.g., at 150 mm perminute, to reposition the gel body 12 for the sintering passes. Someembodiments use the same oxygen partial pressure in the purification andsintering passes.

[0039] After the purification pass, the process 50 sinters the gel bodyby a multi-pass process, e.g., process 30 or 40 of FIG. 2 and 4,respectively (step 58). Sintering passes of the gel body through the hotzone close pores of the gel tube to produce a final transparentovercladding tube.

[0040] The process 50 also prepares a silica-glass core-cladding rod ofhigh optical quality (step 60). The preparation of the core-cladding rodmay proceed by vapor axial deposition (VAD), outside vapor deposition(OVD), or modified chemical vapor deposition (MCVD) as described in U.S.Pat. Nos. 4,217,027; 4,262,035; and 4,909,816, which are incorporated byreference herein.

[0041] After preparation of the core-cladding rod and overcladding tube,the process 50 inserts the core-cladding rod into the overcladding tube(step 62). The process 50 heat collapses the overcladding tube onto thecore-cladding rod, e.g., by heating sections with a torch to 2000° C. ormore, to produce the final preform (step 64). After cooling, the preformis ready for use in optical fiber drawing.

[0042] From a gel body with initial length, OD, and ID of about 1600,120, and 43 mm and produced by the sol-gel process, the process 50produces an overcladding tube with final length, OD, and ID of about1200, 90, and 35 mm, respectively. Thus, the overcladding tubes producedby the process 30 or 40 are useable to construct cylindrical preformswith lengths and ODs greater than 1000 mm and 75 mm, respectively. Somesuch preforms have length and OD greater than about 1200 mm and about 90mm, respectively.

[0043] The sintering process 30 produces overcladding tubes with small Dand CSA variations. For 1000 mm long overcladding tubes, tolerances forID variations along the tube can be kept below 1 percent or between 0.5and 5 percent. CSA variations along the tube can be kept below about 1percent if each sintering pass starts at the bottom end of the cast gelbody and below about 0.5-1.0 percent if successive sintering passesstart at the top end of the cast gel body. In either case, CSAvariations are between about 0.4 to 2.0 percent. Uniformity of ID andCSA reduces dispersion variations along the length of the final opticalfiber.

[0044] The sintering process 30 produces overcladding tubes with lowovalities. Herein, the ovality is defined to be the maximum OD minus theminimum OD at one longitudinal position along the overcladding tube.Ovalities can be equal to or smaller than 100 microns, 50 microns, or 30microns when 90 mm OD overcladding tubes are produced according thesintering process 30. These ovalities produce a variation in OD equal toabout 10⁻¹, 5×10⁻², or 3×10⁻² percent or less at any longitudinalposition along the final preform.

[0045] Variations in a preform's OD result in variations in thecore-cladding geometry of the final fiber. The variations incore-cladding geometry occur, because fiber drawing is controlled by theOD of the preform. Preforms with lower ovalities produce fibers with amore uniform core-cladding ODs. This better control of the core-claddingOD is increasingly important as the ratio of deposited cladding to corematerial decreases.

[0046] Preforms produced by processes 50 and 30 have very low levels of[OH]. For example, levels of [OH] impurities can be below about 2 ppmand even below 0.2 ppm in overcladding tubes produced by the process 30.Optical fibers made with these overcladding tubes can have lightabsorption levels of about 0.4 decibels/kilometer (dB/km) at 1.385microns and levels of about 0.2 dB/km at 1.55 microns. These lowabsorption levels enable using such fibers for optical transmissionapplications over the whole wavelength range between about 1.31 and 1.55microns.

[0047] Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein.

What is claimed is:
 1. A process, comprising: passing a silica gel bodythrough a hot zone under conditions that cause partial sintering of thegel body; and repassing the gel body through the hot zone underconditions that further sinter the gel body.
 2. The process of claim 1,wherein the passing and repassing comprises: vertically moving the gelbody through the hot zone.
 3. The process of claim 1, furthercomprising: passing the gel body through the hot zone under conditionsthat significantly purify the gel body without shrinking the gel body.4. The process of claim 3, further comprising: treating the silica gelbody to cause dehydroxylation prior to performing the passing.
 5. Theprocess of claim 1, wherein the gel body has a tubular shape; thepassing causes at least a 1 percent shrinkage in a diameter of the gelbody; and the repassing causes at least another 1 percent shrinkage inthe diameter of the gel body.
 6. The process of claim 5, wherein one ofthe passing and the repassing causes at least a 5 percent shrinkage ofthe diameter of the gel body.
 7. The process of claim 1, furthercomprising: forming a sol comprising silica particles; and casting thegel body from the sol.
 8. The process of claim 1, wherein both thepassing and repassing include vertically moving the hot zone along thegel body.
 9. The process of claim 8, wherein the passing and repassinginclude regulating a temperature of the hot zone to be at least 1300° C.10. The process of claim 8, further comprising: inserting acore-cladding rod into the further sintered gel body; and heatcollapsing the further sintered gel body onto the rod to produce apreform.
 11. The process of claim 9, wherein the passing and repassingproduce a preform having a level of [OH] impurities of less than 2 partsper million.
 12. The process of claim 1, wherein the repassing includesproducing a transparent silica-glass overcladding tube.
 13. A process,comprising: subjecting one end of a cylindrical silica gel body to a hotzone until the end is at least partially sintered; and verticallypassing the gel body through the hot zone to sinter the gel body bycausing the partially sintered end to enter the hot zone last.
 14. Theprocess of claim 13, wherein the partially sintered end has a diameterat least 1 percent smaller than the diameter of the end prior to thesubjecting.
 15. The process of claim 13, further comprising: producingthe silica gel body from a sol comprising silica particles; and whereinthe gel body has a tubular form.
 16. The process of claim 14, whereinthe passing includes raising the gel body through the hot zone in adirection opposite to the direction of gravity.
 17. The process of claim13, wherein the passing produces a silica glass tube.
 18. A manufacture,comprising: a preform having a central core, a cladding layer, and anovercladding layer; the core, cladding layer, and overcladding layereach comprising silica-glass, the preform having an OD variation of 10⁻¹percent or less at one longitudinal position.
 19. The manufacture ofclaim 18, wherein the length and outer diameter of the preform are atleast as great as 1200 mm and 90 mm, respectively.
 20. The manufactureof claim 18, wherein the overcladding layer has less than 2 parts permillion of hydroxide impurities.
 21. The manufacture of claim 20,wherein the overcladding layer has less than 0.2 parts per million ofhydroxide impurities.
 22. The manufacture of claim 19, wherein thepreform has an OD variation of 5×10⁻² percent or less at onelongitudinal position.
 23. The manufacture of claim 18, wherein theinner diameter of the overcladding layer varies by less than 1 percentover the length of the preform.